DESCRIPTION (Applicant?s Description): Altered DNA methylation patterns, characterized by genome-wide hypomethylation in combination with CpG island hypermethylation, are one of the most common features of human tumors. When a promoter-associated CpG island becomes hypermethylated this can lead to silencing of the gene in a heritable fashion. When this gene acts a key regulator of cell growth then its silencing by improper de novo methylation can lead to a growth advantage for the cell in a manner analogous to mutations and deletions. In normal cells, CPG islands are kept unmethylated by mechanisms which remain almost completely unknown. The objective of this proposal is to gain a better understanding of the mechanisms which regulate de novo methylation in normal cells and how dysregulation of these mechanisms leads to improper CpG island hypermethylation. This will be achieved by the accomplishment of three specific aims. The first will involve characterization of the newly described putative de novo DNA methyltransferases (DNMTs) and a determination of the effect on methylation patterns when these enzymes are overexpressed or inhibited. In the second aim I will examine the role of transcription factor binding and transcription in protecting DNA sequences from de novo methylation. Lastly, a search will be carried out for proteins which interact with the DNMTs using a two-hybrid system. The three aims of this proposal will test the hypothesis that aberrant de novo methylation is a consequence of two factors, (1) increased levels of one or more of the DNMTs and (2) improper DNMT targeting or sequestration resulting from alterations in proteins with which the DNMTs interact. Recent exciting findings related to the role of DNA methylation in DNA repair, chromatin structure, and the discovery of a new family of putative de novo DNMTs provides an excellent starting point for this study and the successful accomplishment of these specific aims should lead to a better understanding of how dysregulation of the normal de novo methylation control mechanisms contributes directly to cancer.